The National Toxicology Program (NTP) utilizes the B6C3F1 hybrid mouse to evaluate the toxicologic and carcinogenic potential of chemicals and other agents. Centralized colonies of C57BL/6N (e84) and C3H/HeN MTV- (e84) mice with homogeneous genetic properties and defined microbial status will ensure an adequate and continuous supply of quality animals for the NTP testing program. The purpose of this contract is to establish, operate and maintain rodent production colonies under strict barrier conditions, in order to produce a large number of disease-free, genetically homogeneous, B6C3F1 mice to be used by testing laboratories performing NTP studies.